Low-loss waveguide and method of making same

ABSTRACT

A method of reducing the scattering losses that involves smoothing of the core/cladding interface and/or change of waveguide geometry in high refractive index difference waveguides. As an example, the SOI-based Si/SiO 2  waveguides are subjected to an oxidation reaction at high temperatures, after the waveguide patterning process. By oxidizing the rough silicon core surfaces after the patterning process, the core/cladding interfaces are smoothened, reducing the roughness scattering in waveguides.

PRIORITY INFORMATION

[0001] This application claims priority from provisional applicationSer. No. 60/217,167 filed Jul. 10, 2000.

BACKGROUND OF TH INVENTION

[0002] The invention relates to the field of optical waveguides, and themethod of manufacturing waveguides.

[0003] Roughness scattering is one of the major sources of transmissionloss in planar waveguides. The roughness at the core/cladding interface,arising from the waveguide patterning process, is responsible for such ascattering. Several methods are possible to reduce the scattering lossesin planar waveguides. Reduction of scattering loss by annealing thewaveguide at high temperature, after the waveguide patterning process,has been previously reported by Kashimura et al. in Japanese Journal ofApplied Physics, Vol. 39, June 2000. This publication reports the lossreduction technique for a waveguide with a low index differencewaveguide between the core and the cladding. GeO₂-doped silica (silicondioxide) waveguides, whose refractive index difference between the coreand the cladding is ˜0.02, were used in that study.

[0004] The roughness scattering is particularly severe for high indexdifference waveguides where the effective refractive index differencebetween the core and the cladding is above 0.1. The effectiverefractiveindex difference higher than 0.1 corresponds to the waveguidesingle-mode cutoff dimension less than roughly 2.5 times the wavelengthin the core. Yet there has been no prior art on reducing the scatteringlosses by subjecting high index difference waveguides to a smoothingprocess after the waveguide patterning.

[0005] A strip Si/SiO₂ waveguide based on SOI is an example of a highindex difference waveguide. A strip waveguide has a core surrounded by acladding comprising one or more materials having different refractiveindices than the core. For SOI waveguides, oxidation at an elevatedtemperature is one method that smoothens rough interface and thusreduces the scattering loss. Smoothing of rough surfaces of siliconafter the patterning process by oxidation, followed by oxide removal,has been reported in the literature. Juan et al., Journal of VacuumScience Technology B, Vol. 14, No. 6, November/December 1996, reportoxidation smoothing of silicon sidewalls for mirror applications whileYahata et al., Japanese Journal of Applied Physics, Vol. 37, July 1998,report smoothing for MOS applications. Yet, there have been nopublications on oxidation smoothing of the silicon waveguide core toreduce scattering losses in strip waveguides.

[0006] U.S. Pat. No. 5,360,982, issued to Venhuizen describes a newwaveguide fabrication technology that produces smooth silicon waveguidesurface. Waveguides with smooth interfaces are formed by local oxidationof the silicon substrate. This process is different from our presentinvention in that the waveguide is formed by oxidation in the patent,while in the invention, the oxidation step is incorporated after thewaveguides are already formed by patterning.

SUMMARY OF THE INVENTION

[0007] The invention provides a technique of making low-loss waveguidesby subjecting the waveguide, after the waveguide patterning process, totreatments that smoothen the core/cladding interfaces, and/or change thewaveguide core dimension. The invention is particularly useful for highindex difference waveguide systems where the scattering loss is high. Inan exemplary embodiment, a method includes smoothing of thecore/cladding interface of SOI-based Si/SiO₂ waveguides by oxidation athigh temperatures, after the waveguide patterning process.

[0008] The invention provides a new waveguide fabrication method thatinvolves a waveguide patterning process, followed by smoothing of thewaveguide core surface. The invention provides a method of reducing thescattering losses in planar waveguide by subjecting thealready-fabricated waveguide to treatments that reduce the dimension ofthe waveguide core, reducing the effective core refractive index,effective refractive index difference, and the scattering losses, sincethe scattering loss is a strong function of effective refractive indexdifference between the core and the cladding.

[0009] The invention shows that the rough silicon core surfaces ofSi/SiO₂ waveguides, resulting from waveguide patterning processes (e. g.photo-lithography and etching), are smoothened by oxidation at hightemperatures. Various oxidants can be used to react with the siliconcore to form SiO₂ on the surfaces at elevated temperatures in Si/SiO₂waveguides.

[0010] The aforementioned smoothing of the waveguide core can beachieved in a diffusive process that tends to minimize the energy of therough surface by annealing the core material, after the waveguidepatterning, at elevated temperatures above 100° C. in a gaseous ambientother than air or vacuum.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a perspective block diagram of an initial SOI platform100 on which a waveguide is formed;

[0012]FIG. 2 is a perspective block diagram of the platform of FIG. 1including a waveguide core 108 after a typical patterning process;

[0013]FIG. 3 is a perspective block diagram of the platform of FIG. 2after the surfaces of the core have reacted with the oxidizing agentsand form a coating layer of SiO₂; and

[0014]FIG. 4 is a perspective block diagram of the platform of FIG. 3following the removal of the SiO₂ layer to show the silicon core surfaceafter smoothing.

DETAILED DESCRIPTION OF THE INVENTION

[0015] An exemplary embodiment of the invention provides an oxidationsmoothing technique that reduces the roughness at the core/claddinginterfaces of Si/SiO₂ waveguide. FIG. 1 is a perspective block diagramof an initial SOI platform 100 on which a waveguide is formed. A topsilicon layer 102 will be made into a waveguide core while a SiO₂ layer104 will become an undercladding layer. A silicon substrate 106 isprovided for mechanical support.

[0016]FIG. 2 is a perspective block diagram of the platform including awaveguide core 108 after a typical patterning process includingphotolithography and etching of the layer 102. The sidewall roughness110 of the core 108 is due to the waveguide patterning process. Thisroughness is responsible for scattering loss in the waveguide. The core108 is then subjected to oxidizing agents, such as O₂ or H₂O gases at anelevated temperature. The surfaces of the core will react with theoxidizing agents and form a coating layer of SiO₂ 112, as shown in FIG.3. Since convex points of the rough surface 110 oxidize faster thanconcave points, the reaction tends to reduce the roughness of the core.

[0017] The reaction rate increases with the reaction temperature. Whenthe reaction temperature is too low, the reaction rate is too slow forenough oxidation. When the reaction temperature is too high, one may nothave a good control over the thickness of SiO₂ formed because of a highreaction rate. In order to grow nm to μm of SiO₂ in a period of minutesto hours, typical temperature ranges between 600 to 1200° C.

[0018] The oxidation time should be chosen carefully to form desiredSiO₂ thickness and to achieve desired waveguide core dimension. Thechoice of time will depend on the oxidation temperature since thereaction rate depends on the temperature.

[0019]FIG. 4 is a perspective block diagram of the platform followingthe removal of the SiO₂ layer 112 to show the silicon core surface 114after smoothing. Alternatively, one can choose not to remove the SiO₂layer 112 since it can act as a cladding layer for the waveguide core inFIG. 3.

[0020] The method of the invention can be used to smoothen the waveguidecore surfaces of other geometries, such as ridge waveguides. Any SOIwaveguide whose core is defined by a patterning process that producessurface roughness can be smoothened by this technique.

[0021] Different oxidants can be used to react with silicon to formSiO₂. The oxidation temperature and time should be chosen according tothe chosen oxidant, since the reaction rate depends on the specificspecies of oxidants used.

[0022] An experiment was carried out to demonstrate the invention. The0.34 μm thick silicon layer of a SOI wafer, which is positioned on topof a 1 μm thick SiO₂ layer, was patterned to get the core of a stripwaveguide. Photolithography and reactive ion etching were used topattern the waveguide core. The waveguide core showed sidewall roughnessresulting from the patterning process. The waveguide went through anoxidation reaction that involved the following steps: a dry oxidationstep for 20 minutes with O₂ gas at 1000° C., a wet oxidation step for 43minutes with H₂O and O₂ at 1000° C., and a dry oxidation step for 20minutes with O₂ gas at 1000° C.

[0023] Most of the SiO₂ was formed during the wet oxidation step, due toits fast reaction, and hence it is a critical step in the experiment.After the reaction the waveguide dimensions were about 0.5 μm in widthand <0.3 μm in height. This single mode waveguide exhibited scatteringloss of less than 0.8 dB/cm, compared to comparably sized waveguide withno oxidation smoothing, which exhibited over 30 dB/cm.

[0024] During the experiment, the waveguide thickness decreased due tothe consumption of silicon to form SiO₂. The reduction in thicknessresulted in the reduction of the effective refractive index of the core,and thus in the reduction of the effective refractive index differencebetween the core and the cladding. The reduction in the effectiverefractive index difference between the core and the cladding resultedin additional reduction of the scattering loss since the scattering lossis a strong function of the refractive index difference between the coreand the cladding.

[0025] While exemplary embodiments of the invention have beenillustrated with subjecting the already-fabricated Si/SiO₂ waveguidecore to the oxidation reaction to reduce the core/cladding interfaceroughness, it will be appreciated that annealing the already-fabricatedSi/SiO₂ waveguide core in an gaseous ambient including hydrogen gases atelevated temperatures smoothens the core/cladding interface, and canalso reduce the roughness and thus reduce losses. The silicon corematerial undergoes a diffusive process that tends to minimize the energyof the rough core surface, smoothing the rough core/cladding interface.

[0026] While exemplary embodiments of the invention have beenillustrated with subjecting the already-fabricated waveguide core to theoxidation reaction to reduce the core/cladding interface roughness, itwill be appreciated that subjecting the already-fabricated waveguidecore to a wet chemical etch smoothens the core/cladding interface, andcan also reduce the roughness and thus reduce losses. Both anisotropicand isotropic etchants can be used. When an anisotropic etchant is usedto smooth a single-crystalline core material, some or all of the coresurfaces can become crystal planes, resulting in atomically smoothsurfaces. Examples of anisotropic etchants for single-crystallinesilicon core are KOH (Potassium Hydroxide) and TMAH(Tetra-Methyl-Ammene-Hydroxide). When an isotropic etchant is used, theetching process reduces the roughness on the core surfaces to minimizethe energy of rough surfaces.

[0027] Although the present invention has been shown and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method of making a low-loss waveguide havingsilicon as its core, comprising: providing a planar strip waveguidehaving core/cladding interface roughness; and subjecting said waveguideto one or more reactions that reduce the core/cladding interfaceroot-mean-square (RMS) roughness in order to in turn reduce scatteringlosses in said waveguide.
 2. The method of claim 1, wherein thewaveguide core is reduced in size.
 3. The method of claim 1, wherein theeffective index of the waveguide is reduced.
 4. The method of claim 1,wherein one of said reactions produces reaction products with differentchemical compositions from that of said core.
 5. The method of claim 4,wherein said reaction products are removed after the reaction.
 6. Themethod of claim 4, wherein said reaction products are left between thecore and the cladding after the reaction.
 7. The method of claim 4,wherein said reaction products have refractive indices that change fromthat of the core to that of the cladding.
 8. The method of claim 4,wherein said reaction products have graded refractive index profilesfrom that of the core to that of the cladding.
 9. The method of claim 1,wherein one of said reactions comprises a wet chemical reaction.
 10. Themethod of claim 9, wherein said wet chemical reaction occurs with one ormore anistropic etchants having OH⁻ ions in an aqueous solution.
 11. Themethod of claim 9, wherein said wet chemical reaction occurs with one ormore isotropic etchants.
 12. The method of claim 1, wherein one of saidreactions comprises a thermal reaction at elevated temperatures above100° C.
 13. The method of claim 1, wherein one of said reactionscomprises an oxidation reaction.
 14. The method of claim 13, whereinsaid oxidation reaction comprises reactant species including oxygen intheir chemical compositions.
 15. The method of claim 13, wherein saidoxidation reaction occurs at temperatures above 600° C.
 16. The methodof claim 13, wherein said reaction products are removed after thereaction.
 17. The method of claim 13, wherein said reaction products areleft between the core and the cladding after the reaction.
 18. Themethod of claim 13, wherein the cladding includes a region of air orvacuum.
 19. The method of claim 13, wherein the cladding includes aregion of air or vacuum before said reactions and no region of air orvacuum after said reactions.
 20. The method of claim 13, wherein thecladding includes a region of material that includes silicon in itschemical composition.
 21. The method of claim 1, wherein one of saidreactions comprises annealing in an ambience other than air at elevatedtemperatures above 100° C.
 22. The method of claim 1, wherein said stripwaveguide has said core surrounded by said cladding: said claddingcomprising one or more materials having different refractive indicesthan said core.
 23. The method of claim 22, wherein the claddingincludes a region of silicon dioxide.
 24. The method of claim 22,wherein the cladding includes a region of air or vacuum.
 25. The methodof claim 22, wherein the cladding includes a region of air or vacuumbefore said reactions and no region of air or vacuum after saidreactions.
 26. The method of claim 1, wherein the cladding includes aregion of material that includes silicon in its chemical composition.27. A method of making a low-loss high index difference waveguide,comprising: providing a planar waveguide containing core/claddinginterface roughness; and subjecting said waveguide to one or moretreatments that reduce the core/cladding interface root-mean-square(RMS) roughness in order to in turn reduce scattering losses in saidwaveguide.
 28. The method of claim 27, wherein the difference in theeffective refractive indices of the core and the cladding of said highindex difference waveguide is greater than or equal to 0.1.
 29. Themethod of claim 27, wherein the single-mode cutoff dimension of saidhigh index difference waveguide is less than 2.5 times the wavelength inthe core.
 30. The method of claim 27, wherein the waveguide core isreduced in size.
 31. The method of claim 27, wherein the effective indexof the waveguide is reduced.
 32. The method of claim 27, wherein one ofsaid treatments is a reaction that produces reaction products withdifferent chemical compositions from that of the core.
 33. The method ofclaim 32, wherein said reaction products are removed after the reaction.34. The method of claim 32, wherein said reaction products are leftbetween the core and the cladding after the reaction.
 35. The method ofclaim 32, wherein said reaction products have refractive indices thatchange from that of the core to that of the cladding.
 36. The method ofclaim 32, wherein said reaction products have graded refractive indexprofile from that of the core to that of the cladding.
 37. The method ofclaim 27, wherein one of said treatments involves wet chemical reaction.38. The method of claim 27, wherein one of said treatments involvesthermal reaction at elevated temperatures above 100° C.
 39. The methodof claim 27, wherein one of said treatments involves oxidation reaction.40. The method of claim 39, wherein said oxidation reaction comprisesthe reactant species including oxygen in their chemical compositions.41. The method of claim 39, wherein said oxidation reaction occurs attemperatures above 600° C.
 42. The method of claim 27, wherein one ofsaid treatments comprises annealing in an ambience other than air atelevated temperature above 100° C.
 43. The method of claim 27, whereinthe core includes silicon in its chemical composition.
 44. The method ofclaim 27, wherein the cladding is a region or regions surrounding thecore with lower effective refractive index than that of the core. 45.The method of claim 44, wherein the cladding includes a region ofsilicon dioxide.
 46. The method of claim 44, wherein the claddingincludes a region of air or vacuum.
 47. The method of claim 44, whereinthe cladding includes a region of air or vacuum before said treatmentsand no region of air or vacuum after said treatments.